1. Field of the Invention
The present invention relates to a method for production of hollow bodies, in particular for radio-frequency resonators.
Radio-frequency resonators comprising a multiplicity of hollow bodies are used in particle accelerators, in particular, which use electric fields to accelerate charged particles to high energies.
In such radio-frequency resonators, also called cavity resonators, an electromagnetic wave is excited which accelerates charged particles along the resonator axis. The particle accelerated in this way experiences a maximum possible energy gain if it travels through the resonator with regard to the phase and the radio-frequency field in such a way that it is situated in the centre of a cavity cell precisely when the electric field strength reaches its maximum there. In this case, the cavity cell length and the frequency are adapted in such a way that the particles experience the same energy gain in each cell. In this case, superconducting resonators for the provision of high field strengths have the advantage that far less energy has to be expended on account of the very low radio-frequency resistance.
2. Discussion of the Prior Art
For a long time one method for resonator production involved the so-called half hollow bodies produced from a polycrystalline niobium metal sheet by means of deep-drawing being connected to one another by electron beam welding. Moreover, DE 37 22 745 A1 discloses a method in which half-cells composed of coated metal sheets are connected. Furthermore, said document discloses a resonator produced according to said method, and in particular a superconducting radio-frequency resonator composed of niobium coated with copper.
Furthermore, U.S. Pat. No. 5,500,995 discloses producing multicell cavity resonators without weld seams by the desired material being applied to a shaping, removable substance, which serves as a support, by means of spinning technology and being correspondingly deformed and the shaping substance subsequently being removed again.
The metal sheets used in the two methods known from the prior art are coated with a suitable superconducting material or completely consist of the latter. In this case, a preferred material is superconducting niobium since it can be machined very well, on the one hand, and has a high critical temperature Tc≅9.2 K and a high critical magnetic field Hc≅200 mT (temperature and magnetic field above which the superconductivity collapses), on the other hand.
After forming, the material is subjected to further treatment in a conventional manner in order to obtain a surface having minimum roughness since the surface is generally roughened during the forming of a polycrystalline material. Moreover, the internal surface is intended to be free of contaminants and impurity particles. This is because surface defects are responsible, inter alia, for the superconductivity collapsing since the currents circulating in the surface layer of the superconductor, which prevent an external magnetic field from penetrating internally (Meiβner-Ochsenfeld effect), are interrupted. Finally, a rough surface results in very high field strengths occurring locally here, which is likewise undesirable.
A customary surface treatment method is a chemical (pickling) method with an acid mixture, referred to as BCP (Buffered Chemical Polishing), using HF (48%), HNO3 (65%) and H3PO4 (85%) in a ratio of 1:1:2. However, since the grain boundaries of polycrystalline material are attacked to a greater extent than the material of the grains themselves, a relatively rough surface is still present after this treatment. Moreover, this method is comparatively time-consuming. A method that yields better results is electropolishing (“EP”), wherein HF and H2SO4 are used in a ratio of 1:9 and an electric field is applied. Electropolishing achieves a very smooth surface even in the case of polycrystalline material, such that a roughness of 250 nm can be achieved in the case of hollow bodies composed of polycrystalline niobium by means of electropolishing.
Since superconductivity is disturbed at the grain boundaries of a polycrystalline material, recently experiments were carried out with regard to the usability of niobium ingots (residual resistivity ratio RRR>250) for production of half-cells with a positive result (P. Kneisel, G. R. Myeni, G. Ciovati, J. Sekutowicz and T. Carneiro; Preliminary Results From Single Crystals and Very Large Crystal Niobium Cavities; Proceedings of 2005 Particle Accelerator Conference, Knoxville, Tenn., USA). In this case, in order to produce a small cavity resonator, two wafers were cut from a coarsely crystalline niobium ingot by means of a wire erosion machine and then brought to the desired form by deep-drawing, without any alteration in the crystalline properties. In that case, too, defect locations occurred, however, at the locations at which the formed crystalline wafers were joined together to form a hollow body.
In addition to the as far as possible defect-free crystal structure in the cavity resonators, it is very important for the quality of superconducting cavity resonators that no superconductivity losses occur at the connection locations as well.
A further factor which has a disturbing effect on superconductivity is hydrogen incorporated in the superconducting material. This problem is conventionally solved by carrying out a thermal treatment.